1. Field of the Invention
This invention relates to a process for the alkoxylation of fatty acid alkyl esters in the presence of a homogeneous catalyst/co-catalyst system.
2. Discussion of Related Art
Alkoxylated alkyl esters, preferably so-called methyl ester ethoxylates, are known nonionic surfactants which have recently acquired considerable interest by virtue of their excellent washing performance. Relevant reviews can be found, for example, in J. Am. Oil Chem. Soc. 56, 873 (1979) and in J. Am. Oil Chem. Soc. 72, 781 (1995).
The addition of alkylene oxides onto compounds containing acidic hydrogen atoms, preferably onto primary alcohols, can be carried out in the presence of various, generally alkaline catalysts. Typical examples are potassium hydroxide or sodium methylate, which are added in the form of alcoholic solutions, or heterogeneous layer compounds of the hydrotalcite type which are introduced into the reaction mixture in the form of solids. By contrast, the insertion of alkylene oxides into the carbonyl ester bond is far more difficult and can only be achieved using special catalysts.
The use of calcined or fatty-acid-modified hydrotalcites for the ethoxylation of fatty acid esters is known from EP-B1 0 339 425 and EP-B1 0 523 089 (Henkel). According to DE-A1 44 46 064 (Lion), the ethoxylation of methyl esters is carried out in the presence of mixed metal oxides which have been surface-modified with metal hydroxides or metal alkoxides. However, these processes have a number of disadvantages. The use of heterogeneous catalysts, i.e. catalysts which are insoluble in the reaction mixture, is technically more complicated because the solid cannot be introduced like a liquid through an automatic metering unit, but generally has to be scooped into the reactor by hand. Removal of the catalyst is also problematical because the catalyst particles are generally so fine that they can only be filtered through special filter candles. However, the catalyst cannot be left in the reaction mixture either because otherwise clouding and sedimentation can occur. From the performance point of view, the results obtained with heterogeneous catalysts are not always satisfactory either. Thus, although the reaction is generally very quick, the products obtained normally have hydroxyl values well below the theoretical value. As a result, products with undesirably high cloud points are obtained. However, since nonionic surfactants only develop optimal washing performance above their cloud point, there is a need for alkoxylated alkyl esters with as low a cloud point as possible.
Accordingly, the problem addressed by the present invention was to provide new catalysts for the alkoxylation of fatty acid alkyl esters which would be free from the disadvantages mentioned above and which, on the one hand, would dissolve in the reaction product and, on the other hand, would give products having improved hydroxyl values and lower cloud points.